The present invention relates to containers for housing radioactive material.
Containers for housing a source of radioactive material are conventionally formed of thick walls made of a shielding material. In some containers, access to the interior of the container employs the use of a tightly fitting removable closure. However, radiation can leak through the gap between the walls and the closure in quantities adequate to present a hazard to the safety of workers in the vicinity of the container.
The Gammacell(trademark) irradiator (MDS Nordion, Kanata, Canada) is an example of a container housing a radioactive source, specifically cobalt 60, at a level of up to 24,000 curies. The irradiator unit is self-shielded, and thus can be installed in a laboratory setting without any further site shielding requirements. The principal use of this irradiator unit is in research and development of applications where high radiation doses are required. Typical applications include development of radiation resistant plastic materials, determination of doses required for sterilization of disposable medical products, and calibration of dosimetry systems.
A conventional irradiator, such as the Gammacell(trademark), utilizes a radioactive source arranged in an annular ring around a central vertical bore within a lead-shielded housing. A cylindrical sleeve comprising a top and bottom drawer and a sample chamber located between the top and bottom drawer is movably located within the central vertical bore along a longitudinal axis. The top and bottom drawer are formed of a shielding material in order to prevent radiation leakage from the bore. The sleeve is capable of movement within the bore in a direction parallel to the longitudinal axis, so that the sample chamber is either in a withdrawn position with respect to the housing where a sample may be loaded into the chamber, or in an inserted position with respect to the housing, whereby the sample chamber is exposed to the radiation source.
To irradiate a sample, the sleeve is first adjusted to the withdrawn position so that the sample chamber is accessible above the housing. A lead shielding collar is opened to allow insertion of a sample into the sample chamber. The collar is then closed and the sample delivery assembly is lowered into the inserted position so that the sample chamber is horizontally aligned with the radiation source.
When in the inserted position the sleeve provides a significant amount of radiation shielding. In order for the sleeve to slide freely in a direction parallel to the longitudinal axis, a gap is present within the bore between the sleeve and the side of the housing facing into the bore. Thus, a worker may be exposed to radiation escaping through this gap and exiting the irradiator unit in the region of the opened shielding collar at the top of the housing.
The disadvantage of the conventional irradiator design is that, due to the position of the radiation source with respect to the gap, radiation emanates through the gap with only minimal scatter. However, the more scattered the radiation is, the less energetic and less hazardous it becomes. Depending on the size of this gap in conventional irradiator units, and its variation with manufacturing tolerances, the radiation fields outside the irradiator, both at the top and the bottom of the unit, can be significant. Measurements of radiation emanating from a conventional irradiator unit indicate that the typical intensity of the radiation beam on the outside of the unit ranges between 500 to 1000 mR/hour (subject to the type of instrumentation used). During installation of a conventional irradiator unit it is estimated that radiation exposure to the hands of the user, from gaps in the unit, ranges between 20 to 200 mrem. While there have been efforts to reduce exposure by minimizing the width of this gap, any further reduction would impede the intended operation of the unit, and could make movement of the sleeve within the bore difficult.
An analogous device is disclosed in U.S. Pat. No. 5,134,295 which is directed to an irradiation apparatus having a source of radiation disposed within a housing, and a chamber for receiving a sample to be irradiated. The housing is closed by means of a removable plug. When a sample is to be irradiated, a valve is opened between the chamber and the radiation source, and the source may then be moved up to irradiate the sample within the chamber. However, escape of radiation from the gap formed at the interface of the housing and the removable plug is not addressed.
The above-discussed prior art relates to removable sleeve assemblies within an irradiator unit, typically comprising a radiation source stored in the irradiator housing, not within the removable sleeve. However, containers for transporting radioactive materials may also comprise a shielded container having a removable closure within which a radioactive source is placed. In the case of such a container, the removable closure acts as a seal for the source and may also provide shielding along with the walls of the container. However, a similar problem as described above with reference to the irradiator unit, exists with containers so configured, in that a gap exists between the removable closure and the walls of the container.
A variety of removable closures have been used in containers for shielding and sealing radioactive material disposed within a central bore of the container. Such closures include containers comprising two lids of different diameters that screw into two threaded portions of the container, for example, U.S. Pat. Nos. 4,893,022; 4,825,088 and 4,595,214. U.S. Pat. No. 4,585,946 is directed to a container for use with radioactive materials wherein the materials are packaged under water. This container provides means for the drainage of water using compressed gas while providing shielding of the radioactive material. U.S. Pat. Nos. 5,042,679 and 4,437,578 disclose containers for storing or transporting radioactive material, and provide a threaded closure as well as gas-tight sealing means.
U.S. Pat. No. 3,132,998 describes a channel shield for use with nuclear reactors. The channels to be shielded are those which allow access to fuel rods within the reactor. To shield a channel, a tubular plug having helical ribs is disposed within the channel. The ribs come into contact with the inner surface of the channel to reduce or eliminate radioactive leakage. However, this plug seals against the wall of the channel, and there is no corresponding structure (e.g. mating groves on the inner channel surface) with which the plug interfaces. The ribs cause scatter and absorption of the radiation emanating from the reactor, thereby reducing radiation leakage from a channel. However, a linear gap still exists where the ribs of the plug contact the corresponding ribs on the inner surface of the channel.
U.S. Pat. No. 5,504,344 describes a radiation shield for use in containers housing a radioactive source. The shield is used to minimize passage of radiation through the gap between a container housing and a closure. The shield involves complementary interdigitating ridges formed on adjacent container components. The ridges may be peaked, rectangular or curved in shape. The interdigitating surfaces of the adjacent components attenuates the amount of radioactivity escaping from the gap between them. However, an uninterrupted upward gap exists at the interface of the interdigitating surfaces, through which radiation may still emanate. Also, once the closure is removed to access the container interior, radiation leakage occurs. However, there is no discussion of the shielding of the gap region of such a container.
The present invention provides a container which obviates or mitigates one or more of the above-noted deficiencies of the prior art. Thus, the invention is directed to a container that attenuates leakage of ionizing radiation from the radioactive source housed therein. The invention provides a radiation shielded container having a removable sleeve and a shielded housing so that the leakage of radioactivity from the gap between the closure and the housing is attenuated, to thereby eliminate or reduce the safety hazard presented to a worker in the vicinity of the container.
According to the invention, there is provided a container for accommodating a radioactive source comprising a shielded housing having a bore extending therein and a removable sleeve adapted for insertion into the bore. The sleeve has a longitudinal axis, and the housing and the sleeve define a gap formed therebetween when the sleeve is inserted into the bore. A first helix is located on a surface of the housing facing into the bore and a second helix located on the sleeve. The first helix mates with the second helix, thereby attenuating leakage of radiation from the container through the gap when a radioactive source is accommodated therein.
Advantageously, the mating helices provide shielding material within the gap between the housing and the sleeve. The helices thus serve to scatter and attenuate the radiation to which a worker may be exposed in the vicinity of the gap, and reduces overall radiation dose received by a worker when in close proximity to the container.